A process for the preparation of a diaryl carbonate is described in U.S. Pat. No. 5,344,954. The document discloses a process in which a dialkyl carbonate is converted in two steps. In a first step the transesterification of the dialkyl carbonate with the aromatic alcohol takes place to yield alkyl aryl carbonate and the corresponding alkyl alcohol. The alkyl alcohol is discharged after the first step and the alkyl aryl carbonate is sent to the second step where disproportionation takes place to yield diaryl carbonate that is recovered at the bottom and dialkyl carbonate that is recovered at the top of the reactor in which the second step is conducted. In the second step further transesterification of the alkyl aryl carbonate with any aromatic alcohol that is still present in the reaction mixture also takes place to yield diaryl carbonate and alkyl alcohol. The dialkyl carbonate and the alkyl alcohol that are recovered at the second step are passed to the first step so that the dialkyl carbonate can be reacted again and the alkyl alcohol can be discharged from the first step.
As disclosed in U.S. Pat. No. 5,344,954 it is known that the transesterification reaction of dialkyl carbonate and aromatic alcohol is an equilibrium reaction. The reaction equilibrium limits conversion, even at long residence times. A shift of the equilibrium can be accomplished by removing reaction product. Therefore the steps are conducted as countercurrent transesterification and reactive distillation.
U.S. Pat. No. 5,344,954 discloses a countercurrent transesterification of dialkyl carbonates with aromatic alcohol, in which transesterification the dialkyl carbonates can be dimethyl and diethyl carbonate. The aromatic alcohol is shown as phenol. The examples show that under comparable reaction conditions the transesterification of dimethyl carbonate results in a higher phenol conversion than the transesterification of diethyl carbonate. For instance, in Examples 2 and 8 similar reaction conditions in the same apparatus were applied in the transesterification of dimethyl carbonate and diethyl carbonate, respectively. Under the conditions shown the phenol conversion was significantly higher in the case of dimethyl carbonate than in the case of diethyl carbonate.
The Examples of U.S. Pat. No. 4,554,110 also disclose a transesterification of dialkyl carbonates with phenol as the aromatic alcohol where the dialkyl carbonates are dimethyl carbonate and diethyl carbonate. The Examples of U.S. Pat. No. 4,554,110 are further discussed below.
It is an object of the invention to provide the skilled person with tools to arrive at a higher conversion of the aromatic alcohol when a di(C2-C4)alkyl carbonate is used as compared to when dimethyl carbonate is used.
It has now surprisingly been found that the conversion of di(C2-C4)alkyl carbonates can be increased more than the conversion of dimethyl carbonate if the catalyst concentration and/or the residence time are chosen such that the product of said catalyst concentration and said residence time (said product being denoted as Pa) is at least 1.5 times the product of catalyst concentration and residence time for the transesterification of the aromatic alcohol with dimethyl carbonate to methyl aryl carbonate and methanol (said latter product being denoted as Pm) at a certain pre-set approach to the equilibrium (or percent of equilibrium) for the latter transesterification reaction. Additional advantages of the present invention are discussed below, including the Examples.